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Plate tectonics is the unifying theory of geology. This important theory explains why Earth's geography has changed through time and continues to change today. It explains why some places are prone to earthquakes and some are not; why some regions have deadly volcanic eruptions, some have mild ones, and some have none at all; and why mountain ranges are located where they are. Plate tectonic motions affect Earth's rock cycle, climate, and the evolution of life. Plate tectonic theory was developed through the efforts of many scientists during the twentieth century.

Before you can learn about plate tectonics, you need to know something about the layers that are found inside Earth. From outside to inside, the planet is divided into crust, mantle, and core. Often geologists talk about the lithosphere, which is the crust and the uppermost mantle. The lithosphere is brittle–it is easily cracked or broken–whereas the mantle beneath it behaves plastically; it can bend. Geologists must use ingenious methods, such as tracking the properties of earthquake waves, to learn about the interior of our planet.

Earth is composed of several layers. On the outside is the relatively cold, brittle crust. Below the crust is the hot, convecting mantle. At the center is the dense, metallic inner core. How do scientists know this? Rocks yield clues, but geologists can only see the outermost rocky layer. Rarely, a rock or mineral, like a diamond, may come to the surface from deeper down in the crust or the mantle. Mostly, though, Earth scientists must use other clues to figure out what lies beneath the planet's surface.

One way scientists learn about Earth's interior is by looking at seismic waves (Figure 6.1). Seismic waves travel outward in all directions from where the ground breaks at an earthquake. There are several types of seismic waves, each with different properties. Each type of wave moves at different speeds through different types of material and the waves bend when they travel from one type of material to another. Some types of waves do not travel through liquids or gases and some do. So scientists can track how seismic waves behave as they travel through Earth and can use the information to understand what makes up the planet's interior. Much more about earthquakes and seismic waves will be presented in the Earthquakes lesson.

Figure 6.1: Different types of seismic waves bend or even disappear as they travel encounter the different properties of the layers that make up Earth's interior. Letters describe the path of an individual P wave or S wave.

Scientists also learn about Earth's interior from rocks from outer space. Meteorites are the remains of the material that the early solar system formed from. Some iron and nickel meteorites are thought to be very similar to Earth's core (Figure 6.2). For this reason they give scientists clues as to the core's makeup and density. An iron meteorite is the closest thing to a sample of the core that scientists can hold in their hands!

Of course, scientists know the most about Earth's outermost layer and less and less about layers that are found deeper in the planet's interior (Figure 6.3). Earth's outer surface is its crust; a thin, brittle outer shell made of rock. Geologists call the outermost, brittle, mechanical layer the lithosphere. The difference between crust and lithosphere is that lithosphere includes the uppermost mantle, which is also brittle.

Figure 6.3: A cross section of Earth showing the following layers: (1) crust (2) mantle (3a) outer core (3b) inner core (4) lithosphere (5) asthenosphere (6) outer core (7) inner core. The lithosphere is made of the crust plus the uppermost part of the mantle. The asthenosphere is directly under the lithosphere and is part of the upper mantle.

The crust is the very thin, outermost solid layer of the Earth. The crust varies tremendously; from thinner areas under the oceans to much thicker areas that make mountains. Just by looking around and thinking of the places you’ve been or seen photos of, you can guess that the crust is not all the same. Geologists make an important distinction between two very different types of crust: oceanic crust and continental crust. Each type has its own distinctive physical and chemical properties. This is one of the reasons that there are ocean basins and continents.

Oceanic crust is relatively thin, between 5 to 12 kilometers thick (3 to 8 miles). This crust is made of basalt lavas that erupt onto the seafloor. Beneath the basalt is gabbro, an igneous intrusive rock that comes from basalt magma but that cools more slowly and develops larger crystals. The basalt and gabbro of the oceanic crust are dense (3.0 g/cm3) when compared to the average of the rocks that make up the continents. Sediments cover much of the oceanic crust, primarily rock dust and the shells of microscopic sea creatures, called plankton. Near shore, the seafloor is thick with sediments that come off the continents in rivers and on wind currents.

Continental crust is much thicker than oceanic crust, around 35 kilometers (22 miles) thick on average. Continental crust is made up of many different rocks of all three major types: igneous, metamorphic, and sedimentary. The average composition of continental crust is about that of granite. Granite is much less dense (2.7 g/cm3) than the basalt and gabbro of the oceanic crust. Because it is thick and has relatively low density, continental crust rises higher above the mantle than oceanic crust, which sinks into the mantle to form basins. When filled with water these basins form the planet's oceans.

Since it is a combination of the crust and uppermost mantle, the lithosphere is thicker than the crust. Oceanic lithosphere is about 100 kilometers (62 miles) thick. Continental lithosphere is about 250 kilometers (155 miles) thick.

Beneath the crust lies the mantle. Like the crust, the mantle is made of rock. The mantle is differentiated from the crust by an increase in rock density as indicated by a sudden increase in seismic wave velocities. Evidence from seismic waves and meteorites let scientists know that the mantle is made of iron and magnesium-rich silicate minerals that are part of the rock peridotite. These types of ultramafic rocks are rarely found at Earth’s surface. One very important feature of the mantle is that it is extremely hot. Although the higher temperatures far exceed the melting points of the mantle rocks at the surface the mantle is almost exclusively solid. The heat in the mantle is mainly due to heat rising from the core. Through the process of conduction, heat flows from warmer objects to cooler objects until all are the same temperature. Knowing the ways that heat flows is important for understanding how the mantle behaves.

Heat can flow in two ways within the Earth. If the material is solid, heat flows by conduction, and heat is transferred through the rapid collision among atoms. If a material is fluid and able to move—that is, it is a gas, liquid, or a solid that can move (like toothpaste)—heat can also flow by convection. In convection, currents form so that warm material rises and cool material sinks. This sets up a convection cell (Figure 6.4).

Convection occurs when a pot of water is heated on a stove. The stove heats the bottom layer of the water, which makes it less dense than the water above it, so the warmer bottom water rises. Since the layer of water on the top of the pot is not near the heat source, it is relatively cool. As a result, it is denser than the water beneath it and so it sinks. Within the pot, convection cells become well established as long as there is more heat at the bottom of the pot than on the top.

Convection cells are also found in the mantle. Mantle material is heated by the core and so it rises upwards. When it reaches the surface of the Earth, it moves horizontally. As the material moves away from the core's heat, it cools. Eventually the mantle material at the top of the convection cell becomes cool and dense enough that it sinks back down into the deeper mantle. When it reaches the bottom of the mantle, it travels horizontally just above the core. Then it reaches the location where warm mantle material is rising, and the mantle convection cell is complete. The relationship between mantle convection and plate tectonics will be discussed in the final section of this chapter.

At the planet's center lies a dense metallic core. Scientists know that the core is metal for two reasons: The first is that some meteorites are metallic and they are thought to be representative of the core. The second is that the density of Earth's surface layers is much less than the overall density of the planet. We can calculate Earth's density using our planet's rotation. If the surface layers are less dense than the average for the planet, then the interior must be denser than the average. Calculations indicate that the core is about 85% iron metal with nickel metal making up much of the rest. These proportions agree with those seen in metallic meteorites. Seismic waves indicate that the outer core must be liquid and the inner core must be solid.

If Earth's core were not metallic, the planet would not have a magnetic field. Metal conducts electricity, but rock—which makes up the mantle and crust—does not. The best conductors are metals that can move, so scientists assume that the magnetic field is due to convection in the liquid outer core. These convection currents form in the outer core because the base of the outer core is heated by the even hotter inner core.

List two ways that scientists learn about what makes up the planet’s interior.

What types of rock make up the oceanic crust?

What types of rock make up the continental crust?

List two reasons that scientists know that the outer core is liquid.

Describe the properties of each of these parts of the Earth's interior: lithosphere, mantle, and core. What are they made of? How hot are they? What are a few of their physical properties?

Suppose that Earth's interior contains a large amount of lead. Based on your prior knowledge, how dense is lead? Would the lead be more likely to be found in the crust, mantle, or core?

When you put your hand above a pan filled with boiling water, does your hand warm up because of convection or conduction? If you touch the pan, does your hand warm up because of convection or conduction? Based on your answers, which type of heat transfer moves heat more easily and efficiently?